Lifetime of thecΠu3Metastable State ofH2,D2, and HD

Abstract

The lifetime of the $v=0\mathrm{}$ $c{}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ metastable state of ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, and HD has been measured using the time-of-flight technique. The velocity distribution of a thermal beam of metastable molecules is sampled and detected at two positions, 1.9 and 6.7 m from the pulsed electron gun used to excite the ground-state molecules effusing from a source slit. A comparison of the number of metastables within specific velocity intervals at the two detectors determines the number which decay in flight and yields an experimental plot of the number which decay versus time of flight. The lifetime $\tau$ is then obtained from the slope ($=-\frac{1}{\tau }$) of a straight line least-squares fitted to the decay plot. The result, $\tau =1.02\mathrm{}\pm 0.05$ msec, is the same for both para-and ortho-${\mathrm{H}}_2$, as well as for ${\mathrm{D}}_2$ and HD. Since all three isotopic combinations have the same measured lifetime, and since the lifetime for a forbidden predissociation through spin-orbit mixing with the ${}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state would be expected to depend upon details of the fine structure, our value is the radiative lifetime for a combination of magnetic-dipole electric-quadrupole decay of the $v=0\mathrm{}$ $c{}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state to the repulsive ${}_{}{}^3{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ state.